Human speech lies in the frequency range of approximately 7 Hz to 10 kHz. Because traditional telephone systems only provide for the transmission of analog audio signals in the range of about 300 Hz to 3400 Hz or a bandwidth of about 3 kHz (narrowband speech), certain characteristics of a speaker's voice are lost and the voice sounds somewhat muffled. A telephone system capable of transmitting an audio signal approaching the quality of face-to-face speech requires a bandwidth of about 6 kHz (wideband speech).
Known digital transmission systems are capable of transmitting wideband speech audio signals. However, in order to produce an output audio signal of acceptable quality with a bandwidth of 6 kHz, these digital systems require a transmission channel with a transmission rate that exceeds the capacity of traditional telephone lines. A digital system transmits audio signals by coding an input audio signal into a digital signal made up of a sequence of binary numbers or bits, transmitting the digital signal through a transmission channel, and decoding the digital signal to produce an output audio signal. During the coding process the digital signal is reduced or compressed to minimize the necessary transmission rate of the signal. One known method for compressing wideband speech is disclosed in Recommendation G.722 (CCITT, 1988). A system using the compression method described in G.722 still requires a transmission rate of at least 48 kbit/s to produce wideband speech of an acceptable quality.
Because the maximum transmission rate over traditional telephone lines is 28.8 kbit/s using the most advanced modem technology, alternative transmission channels such as satellite or fiber optics would have to be used with an audio transmission system employing the data compression method disclosed in G.722. Use of these alternative transmission channels is both expensive and inconvenient due to their limited availability. While fiber optic lines are available, traditional copper telephone lines now account for an overwhelming majority of existing lines and it is unlikely that this balance will change anytime in the near future. A digital phone system capable of transmitting wideband speech over existing transmission rate limited telephone phone lines is therefore highly desirable.